Assembly of coaxial pipes including junction parts with sealed inner cavities, and method for making same

Abstract

An assembly of at least two coaxial pipes having at least two coaxial pipe assembly unit elements, one of which has, at one end, a first junction part assembled to a second junction part at the end of another said coaxial pipe assembly unit element, each of said first and second junction parts being a body of revolution, wherein the ends of the two non-welded-together inner second branches of said first and second junction parts are in metal-on-metal contact one on the other under elastic deformation stress via their contact surface as a result of the shrinking of the weld between the ends of the two outer second branches of the junction parts, such that the inner second cavities are leaktight relative to the inside of the inner pipe.

The invention claimed is: 
     
       1. An assembly of at least two coaxial pipes constituted in a main portion by an outer pipe and an inner pipe defining an annular space preferably containing lagging material, the assembly being constituted by assembling together unit elements of at least two coaxial pipes, comprising at least two coaxial pipe assembly unit elements, one of which has, at at least one end, a first junction part joined to a second junction part at the end of another said coaxial pipe assembly unit element, each of said first and second junction parts being a body of revolution defined as follows:
 in a radial direction relative to a longitudinal axis XX′ of symmetry of said part, it is defined by a cylindrical inner wall of substantially the same diameter as the intermediate portion of said inner pipe, and by a cylindrical outer wall of diameter substantially equal to the outside diameter of the intermediate portion of said outer pipe; and 
 in said direction of the longitudinal axis XX′
 on the end of said junction part that is joined by welding to the ends of said outer and inner pipes of a said unit element of an assembly of at least two coaxial pipes, said outer and inner walls of said junction part forming respective outer and inner first branches in longitudinal section, said outer and inner first branches defining a first annular cavity; and 
 at the opposite end of said junction part where it is joined to another said junction part, itself connected by welding to the end of said another unit element of an assembly of two coaxial pipes, said outer and inner walls forming in longitudinal section respective outer and inner second branches defining an internal second annular cavity;
 the two said first and second junction parts being welded to each other solely via the ends of their said outer second branches; 
 the bottoms of said first and second cavities being spaced apart in said longitudinal direction XX′ so as to define a massive solid zone of said junction part in which said outer and inner walls form the outer and inner faces of a single cylindrical wall; and 
 
 
 the ends of said two non-welded-together inner second branches of said first and second junction parts are in metal-on-metal contact with each other under elastic deformation stress via their contact surface, which is a surface of revolution, between an outer first surface of revolution on the outer face of the inner second branch of the first junction part and an inner second surface of revolution on the inner face of the inner second branch of the second junction part, said inner second branch of the first junction part forming a male inner second branch situated inside the female inner second branch of the second junction part, 
 wherein said inner second surface of revolution is of a shape that is different, but of the same axis of revolution as the outer first surface of revolution, such that said outer first surface of revolution and inner second surface of revolution present, when they are arranged coaxially and engaged in contact one against the other, an initial line of contact that is circular and that transforms into said contact surface after said elastic deformation stress resulting from the elastic deformation due to the shrinkage of the weld between the ends of said outer second branches of said two junction parts, so that said internal second cavities are leaktight relative to the inside of the inner pipe. 
 
     
     
       2. An assembly of at least two coaxial pipes according to  claim 1 , wherein:
 said inner second branch of said first junction part forms a male inner second branch inside the female inner second branch of the second junction part, and said male inner second branch is of stiffness that is less than the stiffness of the female inner second branch; and 
 said outer first surface of revolution and said inner second surface of revolution are in metal-on-metal contact resulting from elastic deformation on a said contact surface, which surface presents a dimension in projection onto the axial longitudinal direction that is less than the dimension in projection onto the axial longitudinal direction of the overlap zone between said male inner second branch and said female inner second branch. 
 
     
     
       3. An assembly of at least two coaxial pipes according to  claim 1 , wherein the length of the generator line of said contact surface of revolution between said outer first surface of revolution and said inner second surface of revolution lies in the range 0.5 mm to 5 mm. 
     
     
       4. An assembly of at least two coaxial pipes according to  claim 1 , wherein said inner second surface of revolution is not inscribed in the same frustoconical envelope as said outer first surface of revolution. 
     
     
       5. An assembly of at least two coaxial pipes according to  claim 1 , wherein said outer first surface of revolution is a frustoconical surface having a half-angle at the apex α 1  that is less than the half-angle at the apex α 2  of said frustoconical inner second surface of revolution, where α 1  and α 2  are less than 45°. 
     
     
       6. An assembly of at least two coaxial pipes according to  claim 5 , wherein the thickness of said male inner second branch of said first junction part decreases going from the bottom, of said annular second cavity to its longitudinal end. 
     
     
       7. An assembly of at least two coaxial pipes according to  claim 6 , wherein the longitudinal end of said male inner second branch of said first junction part includes at its longitudinal end beside its inner face a chamfered termination inclined at an angle β relative to the axial longitudinal direction, the angle of inclination β of said chamfered end being greater than the half-angle at the apex α 1  of said frustoconical outer first surface of revolution. 
     
     
       8. An assembly of at least two coaxial pipes according to  claim 1 , wherein said outer first surface of revolution presents a frustoconical shape continuing at the inner end of said inner second branch in the form of a toroidal surface having the same axis of revolution XX′, with its convex side facing towards said inner second surface of revolution of frustoconical shape. 
     
     
       9. An assembly of at least two coaxial pipes according to  claim 1 , wherein the end of the inner second branch of the first junction part is set back in said axial longitudinal direction relative to the end of the outer second branch of the first junction part, and the end of the outer second branch of the second junction part is set back in the axial longitudinal direction relative to the end of the inner second branch of the second junction part, which is of stiffness that is greater than the stiffness of the inner second branch of the first junction part. 
     
     
       10. A method of assembling an assembly of at least two coaxial pipes according to  claim 1 , the method comprising:
 1) assembling together a first coaxial pipe assembly, having a said first junction part at its upstream end, and a unit element of a second said coaxial pipe assembly, having at its downstream end, a second junction part different from said first junction part, whereby the first contact of said second junction part with said first junction part takes place via a circular line of contact between said outer first surface of revolution and said inner second surface of revolution, said two outer second branches of said first and second junction parts, the front end faces of said outer second branches of said first and second junction parts being spaced apart by a distance of at least 1 mm; and 
 2) making a weld between the end faces and the chamfers at the free ends of said outer second branches only of said first and second junction parts of said two pipe unit elements.